I found a bug when I was trading to the villager.

When I was testing my worlds yesterday,I found a bug when I was trading to the Villager.
Some items can not be stacked,you know,such as the lava bucket.But when I was trading to a villager who wanted to trade lava buckets.
And when I try to trade,I find I can stack it.And I now the it is a bug,so I reported to you.